This question usually means "how do I make a hollow object, like a waterglass, a jug, etc".

Before answering that question, let me explain some things about how POV-Ray handles objects:

Although the POV-Ray documentation talks about "solid" and "hollow" objects, that is not how it
actually works. "Solid" and "hollow" are a bit misleading terms to describe the objects. You can
also make an object "hollow" with that same keyword, but it is not that simple.

Firstly: POV-Ray only handles surfaces, not solid 3D-objects. When you specify a sphere, it is actually just a
spherical surface. It is only a surface and it is not filled by anything. This can easily be seen by putting the
camera inside the sphere or by clipping a hole to one side of the sphere with the clipped_by keyword (so you can look
inside).

People often think that POV-Ray objects are solid, really 3D, with solid material filling the entire object because
they make a 'difference' CSG object and it seems like the object is actually solid. What the 'difference' CSG actually
does is to cut away a part of the object and add a new surface in the place of the hole, which
completely covers the hole, so you cannot see inside the object (this new surface is actually the part of the second
object which is "inside" the first object). Again, if you move the camera inside the object, you will see
that actually it is hollow and the object is just a surface.

So what is all this "solid" and "hollow" stuff the documentation talks of, and what is the
"hollow" keyword used for?

Although objects are actually surfaces, POV-Ray handles them as if they were solid. For example, fog and media do
not go inside solid objects. If you put a glass sphere into the fog, you will see that there is no fog inside the
sphere.

If you add the "hollow" keyword to the object, POV-Ray will no longer handle it as solid, so fog and
atmosphere will invade the inside of the object. This is the reason why POV-Ray issues a warning when you put the
camera inside a non-hollow object (because, as it says, fog and other atmospheric effects may not work as you
expected).

If your scene does not use any atmospheric effect (fog or media) there is not any difference between a
"solid" or "hollow" object.

So all the objects in POV-Ray are hollow. But the surface of the objects is always infinitely thin, and there is
only one surface. With real world hollow objects you have always two surfaces: an outer surface and an inner surface.

Usually people refer to these kind of objects when they ask for hollow objects. This kind of objects are easily
achieved with a 'difference' CSG operation, like this: